1. Field of the Invention
The present invention relates to a charging circuit for an electronic flash device.
2. Description of Related Art
Known methods for rapidly charging electronic flash devices include a method of increasing the turn ratio between the primary winding and the secondary winding of an oscillation transformer and detecting the voltage of a main capacitor so as to stop the charging voltage at a predetermined voltage at which the electronic flash device becomes usable, and a method of switching the turn ratio between the primary winding and the secondary winding of an oscillation transformer on the way during charging. Another method which has been used for rapid charging involves charging with a push-pull converter (hereinafter referred to as "P--P converter").
FIG. 4 illustrates a known P--P type DC--DC (direct-current-to-direct-current) converter circuit. Reference numeral 1 denotes a battery which is the electrical power source, reference numeral 2 denotes a resistor, reference numeral 3 denotes a capacitor, reference numeral 4 denotes a first oscillation transistor, reference numeral 5 denotes a first switching element, and reference numeral 6 denotes a diode. A parallel circuit composed of the resistor 2 and the capacitor 3 is connected between the base of the first oscillation transistor 4 and the emitter thereof, and the base of the first oscillation transistor 4 is connected to the cathode of the diode 6 via the first switching element 5, and the anode of the diode 6 is connected to the negative pole of the battery 1.
Reference numeral 7 denotes a resistor, reference numeral 8 denotes a capacitor, reference numeral 9 denotes a second oscillation transistor, reference numeral 10 denotes a second switching element, and reference numeral 11 denotes a diode. A parallel circuit composed of the resistor 7 and the capacitor 8 is connected between the base of the second oscillation transistor 9 and the emitter thereof, and the base of the oscillation transistor 9 is connected to the cathode of the diode 11 via the second switching element 10, and the anode of the diode 11 is connected to the negative pole of the battery 1.
Reference numerals 13 and 14 denote resistors. Reference numeral 15 denotes an oscillation transformer, having a first primary winding P.sub.1, a second primary winding P.sub.2, a secondary winding S and a feedback winding F. The junction point of the first and second primary windings P.sub.1 and P.sub.2 is connected to the negative pole of the battery 1, and the other ends of the first and second primary windings P.sub.1 and P.sub.2 are connected to the collector of the first oscillation transistor 4 and the collector of the second oscillation transistor 9, respectively.
Both ends of the secondary winding S are connected to each other via the diodes 18 and 19. The feedback winding F is connected between the cathodes of the diodes 6 and 11 according to the polarity indicated in FIG. 4, and the resistors 14 and 13 are connected to the diodes 6 and 11 in a parallel manner. The junction point of the cathodes of the diodes 18 and 19 is connected to the positive pole of a main capacitor 23.
Reference numeral 16 and 17 denote diodes, which are inserted between the cathode of the diode 11 and the anode of the diode 18 and between the cathode of the diode 6 and the anode of the diode 19, respectively. Reference numeral 20 denotes a voltage detecting circuit, which is connected to the main capacitor 23 in a parallel manner. Reference numeral 21 denotes a trigger circuit for triggering emission of light, and reference numeral 22 denotes a discharge tube. Reference characters "a", "b" and "c" denote terminals, which are connected to a camera control circuit (not shown).
Now, description of the DC--DC converter circuit will be made. When a charging signal for the electronic flash device is supplied from the camera control circuit (not shown), a voltage of a high level is generated at the terminal "a". Accordingly, the level of each of the resistor 12, the control electrode of the first switching element 5 and the control electrode of the second switching element 10 becomes high, so that the first and second switching elements 5 and 10 both are brought into a conducting state. Since the structures of the primary-side oscillation circuits respectively composed of the first oscillation transistor 4 and the second oscillation transistor 9 are the same, the start of oscillation is effected contingently to the balance of the various elements. Here, description will be made under the presupposition that the first oscillation transistor 4 is the first to start oscillation.
Once the switching elements 5 and 10 are brought into a conducting state, a base current flows from the battery 1 via the emitter and base of the first oscillation transistor 4, the first switching element 5 and the resistor 14. Due to the base current, a collector current of h.sub.FE times the base current flows to the first primary winding P.sub.1 of the oscillation transformer 15. The collector current causes an electromotive force to be generated in the secondary winding S and the feedback winding F.
Due to the electromotive force generated in the secondary winding S, a current flows through the diode 18, the main capacitor 23, the battery 1, the emitter and base of the first oscillation transistor 4, the first switching element 5 and the diode 17, and due to the electromotive force generated in the feedback winding F, a current flows through the resistor 13, the battery 1, the emitter and base of the first oscillation transistor 4 and the first switching element 5, so that with each current flowing as the base current of the first oscillation transistor 4, the first oscillation transistor 4 instantly comes into a saturated state.
When the current flows to the first primary winding P.sub.1 and the magnetic flux of the core thereof becomes saturated, a reverse electromotive force is generated in each of the windings, so that a reverse bias is applied between the base and emitter of the first oscillation transistor 4, and a current flows to the feedback winding F via the resistor 14, battery 1, the emitter and base of the second oscillation transistor 9 and the second switching element 10. Also, when the current flows to the second primary winding P.sub.2, a current flows through the diode 19, the main capacitor 23, the battery 1, the emitter and base of the second oscillation transistor 9, the second switching element 10 and the diode 16, so that with each current becoming the base current of the second oscillation transistor 9, the collector current of the second oscillation transistor 9 flows to the second primary winding P.sub.2. This electromotive force causes the electromotive force generated in the secondary winding S and the feedback winding F to be generated at the polarity at which the electromotive force increases as a base current, and thus the second oscillation transistor 9 instantly comes into a saturated state.
In such a way as described above, the first and second oscillation transistors 4 and 9 are instantly switched, and alternately are placed in conducting states and non-conducting states, thereby performing DC--DC converting operations. This voltage step-up operation causes a high-voltage charging charge to be stored in the main capacitor 23. Then, the potential of the main capacitor 23 increases, and a charge completion signal is generated at the point of time at which the voltage detecting circuit 20 judges the voltage to be a certain voltage, e.g., 330V. The charge completion signal is supplied via the connection terminal "b" to the camera control circuit (not shown). The camera control circuit changes the level of the connection terminal "a" from a high level to a low level so as to place the switching elements 5 and 10 in a non-conducting state, and stops the oscillation of the oscillating transistors 4 and 9 by means of stopping the base current thereof, so that charging to the main capacitor 23 is stopped. Subsequently, at the point of time at which a flash emission signal for the electronic flash device is supplied to the trigger circuit 21 from the camera control circuit via the terminal "c" in accordance with a photo-taking operation, a high-voltage pulse of several kV is applied to the trigger electrode 22a of the discharge tube 22 from the trigger circuit 21. Then, the discharge tube 22 is excited by this high-voltage pulse, and the charged charge of the main capacitor 23 is discharged so as to cause the discharge tube 22 to emit flash light, thus illuminating an object to be photographed.
According to such a known example, in the event that a known single oscillation circuit is used, there is approximately 10 .mu.sec of time until the magnetic saturation of the oscillation transformer is cleared, and although the duty ratio between the conducting state and non-conducting state differs between at the time of start of oscillation and at the time of completion of charging, that duty ratio taken as an overall average is around 70% in general electronic flash devices built in cameras. Accordingly, with the P--P converter shown in the known example in FIG. 4, that duty ratio can be made greater by instantly inverting the operation of the oscillation transformer, thus allowing for charging of the main capacitor 23 to be perpetually conducted, thereby facilitating speedy charging.
However, although the duty ratio of the known example is increased, switching time of 3 .mu.sec to 5 .mu.sec is required for switching of the oscillation transistors 4 and 9, so that the duty ratio is around 85%, meaning that the duty ratio could not be made to unlimitedly approach 100%.